Owning to environmental concerns, there is an ever present need to reduce emissions from internal combustion engines. Of particular interest herein are internal combustion engines operated using a lean air/fuel mixture, known as “lean-burn engines.” A common lean-burn engine is a diesel engine. The emissions in the exhaust gas of a lean-burn engine can be divided into two groups—primary and secondary emissions. Primary emissions involve pollutant gases which are formed directly by the combustion process of the fuel in the engine and are present in the raw emission before passing through exhaust gas purification devices. The exhaust gas of lean-burn engines comprises the usual primary emissions of carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NOx), and soot (also known as particulate matter or PM), together with a relatively high oxygen content of up to 15% by volume. Secondary emissions are pollutant gases which can be formed as by-products in the exhaust gas purification units. Such secondary emissions may include, for example, “slip” ammonia (NH3) and NOx as discussed below.
Emission control systems have various configurations. For example, referring to FIG. 1, a typical emissions control system 100 for a diesel engine is shown. Immediately after the exhaust gas leaves the engine (not shown), a diesel oxidation catalyst (DOC) 101 oxidizes primary pollutants such as unspent fuel (hydrocarbons) and carbon monoxide to render them harmless. Other primary pollutants such as NOx cannot be oxidized, but instead must be reduced to nitrogen. Reducing NOx, however, tends to be more difficult because of the high oxygen content in the exhaust stream.
A known method of removing NOx from exhaust gases in the presence of oxygen is the process of selective catalytic reduction (SCR). SCR uses ammonia as a reducing agent over a suitable catalyst, SCR catalyst 103 as shown in FIG. 1. The reducing agent is introduced into the exhaust gas train using an injection nozzle 102. In place of ammonia, a compound which can readily be decomposed into ammonia, for example, urea can be used for this purpose.
To ensure complete reduction of the NOx, ammonia has to be added to the exhaust gas in at least a stoichiometric ratio to the nitrogen oxides, and excess ammonia is preferred to improve the conversion of NOx. Excess ammonia, however, significantly increases the risk of ammonia slipping past the SCR catalyst, and becoming a secondary emission. Ammonia that breaks through or slips past the SCR catalyst is referred to as “slip ammonia.” Since ammonia is a gas which has a penetrating odor even in low concentrations, it is desirable to minimize slip ammonia. However, the precise metering of ammonia is difficult in internal combustion engines in motor vehicles because of the fluctuating operating conditions of motor vehicles (e.g., acceleration/deceleration). Therefore, inevitably excess ammonia will be injected into the system, resulting in significant ammonia slip downstream of the SCR catalyst.
The system 100 also comprises a Diesel Exotherm Catalyst (DEC) 105 behind the SCR 103 to facilitate periodic exothermic reactions to generate heat sufficient to regenerate the soot from a Catalyzed Soot Filter (CSF) 106. To this end, a hydrocarbon injector 104 is located just upstream of the DEC. The injector 104 injects fuel or HCs into the exhaust steam when the exhaust temperature is above the DEC light-off temperature. The DEC then oxidizes the HCs to generate an exotherm, which, in turn, heats the filter to clean the soot from it. Because the DEC is located behind the SCR, the SCR does not experience the detrimental high temperatures associated with removing the soot.
Although the DEC is good at oxidation, it has the potential to also unselectively oxidize any ammonia slip from the SCR catalyst and convert it to NOx, thereby increasing the NOx emissions. To counter this, one approach involves using an Ammonia Slip Catalyst (ASC) 201 as shown in FIG. 2. The ASC is selective for removing NH3 with minimal oxidation to NOx after the SCR catalyst 103, but in front of the HC injector 104. Typically the ASC has a low platinum group metals (PGM) loading (e.g., 0.5 to 10 g/ft3) to maximize the selectivity to N2. The disadvantage of this system is the additional catalyst volume required for the NH3 slip catalyst in an already large emissions control system 200.
Therefore, Applicant recognizes the need for a simplified exhaust system that eliminates ammonia slip while heating the exhaust stream periodically to regenerate soot from the soot filter. The present invention fulfills this need among others.